1. Field of the Invention
This invention relates to nuclear reactor plant systems, and more particularly provides an improved valve arrangement for a nuclear reactor plant Residual Heat Removal System.
2. Description of the Prior Art
Commercial electric power generating nuclear reactor plants have demonstrated an unparalleled safety record. It has continuously been the history of the industry, including the manufacturers, utilities and the regulatory agencies to improve not only the safety-oriented systems within the plants, but also auxiliary systems. Many of the design bases for the plant systems are defined in terms of specific hypothetical accident occurrences. Also, more general hypotheses are also applied, such as those referred to as "common mode failure" and "single failure" criteria. The latter, "single failure", refers to protection of the function of plant systems from an assumed failure in any single component in a given system. Such assumed failures may be either passive, such as a piping rupture, or active, such as failure of a valve to respond to a signal to open or close. Typical design response to such assumptions are redundancy in system design and component number, and also system physical separation. Similar criteria have also been applied so as to result in design requirements that certain plant functions must be able to be initiated and controlled from the plant control room.
A typical nuclear plant comprises a primary reactor coolant system housed in a sealed containment structure and a plurality of auxiliary systems, located both within and without the containment, which perform auxiliary functions such as waste processing, component cooling, and emergency coolant injections. A typical residual heat removal auxiliary system (RHRS) assists in cool-down and start-up of the reactor primary system, as well as maintaining the plant in a cold condition during refueling and maintenance operations, and also assisting in emergency injections.
In order to perform these functions, the RHR system includes valves, pumps, connecting piping and electrical sources and interlocks, portions of which are located both within and without the containment. Because the system penetrates the containment, passes reactor coolant, provides a cool-down function, and is directly connected to the primary reactor coolant system, certain of the accident assumptions have been applied to prior art RHR systems and have resulted in good, reliable and safe designs. Typical of such designs is a two-train system. Each physically separate train includes piping which connects to the primary reactor coolant system and contains two motor-operated isolation valves in series; the piping then passes through the containment wall to a pump and then a heat exchanger, and then returns again through the containment wall and connects to the primary reactor coolant system. The two motor-operated valves in series receive electrical power from separate sources, and are individually interlocked with pressure transmitters in the reactor coolant system. In other terms, the first train contains a valve connected to a first electrical bus and also a valve connected to a second bus. The second train similarly contains a valve conected to the same first bus, and a valve connected to the same second bus. With such a system, it can be seen that if one assumes loss of electrical power from one of the sources, when, for example, the valves are closed, neither train can function until the loss is corrected or the valves, located within the containment, are manually opened. For example, if the valves are all closed as is typical during reactor power operation, and it is desired to open them for cool-down, loss of electrical power to one bus prevents opening of either train. The single failure assumption of an electric bus upon which the example is based is not inconsistent with the policies of regulatory authorities, such as the Nuclear Regulatory Commission. Therefore, a system which overcomes the exemplary deficiencies and will allow functioning of a residual heat removal train under such assumed circumstances, will enhance the operational reliability of commercial nuclear power plants.